Generally, a semiconductor memory device can be classified as a volatile memory device or a non-volatile memory device. A volatile memory device may lose its stored information when no power is applied, while a non-volatile memory device may retain its stored information even when there is no power. A flash memory device is a non-volatile memory device. Types of flash memory devices include NOR flash memory devices and NAND flash memory devices. A NOR flash memory device can independently control each memory cell, and thus, may have a relatively fast operational speed. However, because a NOR flash memory device may require at least one contact for every two cells, such a device may have a relatively large cell area. In contrast, a NAND flash memory device can control one string including a plurality of memory cells, and thus, may be advantageously used to achieve greater device integration.
A NAND flash memory device may include a substrate that has an active region extending in a predetermined direction. A string select line and a ground select line may be disposed on the substrate to cross over the active region, and a plurality of word lines may be disposed between the select lines. Source/drain regions may be disposed on the active region between the word lines and the select lines.
However, as memory devices have become more highly integrated, an area of the source/drain regions may be reduced. Accordingly, reliability and/or operational characteristics of NAND flash memory devices may be deteriorated. For example, if a program voltage or a pass voltage is applied to word lines to prevent program inhibit of a memory cell connected to a bit line, an electric field of a channel may be increased. Therefore, a depletion layer or region of the channel may expand, and a depletion layer of the source/drain region may also expand. Accordingly, an effective area of the source/drain region may be reduced. If an area of the source/drain regions becomes sufficiently small, the channels of the memory cells may be disconnected due to the reduction in the area of the source/drain regions.